Add support for multiple endpoints

When running etcd3 as a cluster, it'd be useful to be able to specify multiple endpoints to try to connect to. It seems the Go client allows that, for example.

Start by tracing how the Python client establishes its etcd3 connection and reviewing any existing connection tests. Use the Go client's multiple-endpoint behavior as a reference, then verify that a cluster can be configured with multiple endpoints and that connection attempts behave as expected.

Written by the indexing model from the issue text.